1. Field of the Invention
This invention relates to a varactor tuning apparatus for a strip transmission line device and more particularly to an apparatus for tuning a device having a biased active element in a strip transmission line by electrically varying varactor capacitance in an independent varactor circuit RF coupled to the strip transmission line of the device.
2. Description of the Prior Art
In oscillators and other electronic devices which utilize strip transmission lines, it is often required to tune such devices to various different frequencies within a certain frequency range of intended operation. The term strip transmission line refers to that form of open transmission line which includes a substrate of dielectric material with a narrow conductive strip and a substantially wider ground conductor disposed on the substrate. The strip transmission line may be in either (i) the asymmetrical configuration, called a "microstrip," using a single ground plane on one side of the substrate and a narrow conductive strip on the opposite side of the substrate, (ii) the symmetrical configuration, commonly called a "stripline," having two ground planes on opposite sides of a narrow conductive strip, each ground plane being spaced from the narrow strip by a dielectric layer, or (iii) a surface strip transmission line configuration described by C. P. Wen in the IEEE 1969 G-MTT (Group on Microwave Theory and Techniques) International Microwave Symposium Digest, May 1969, under the title of "A Surface Strip Transmission Line for Nonreciprocal Gyromagnetic Device Applications." In the surface strip transmission line configuration, the narrow conductor is spaced a short distance from the wider conductor and both are on the same surface of the dielectric substrate. The tuning of the device to the desired frequencies has been achieved in many prior art devices by mechanical means. Such means function to change the capacitance in a circuit by varying the lengths or areas of capacitive materials or by adjusting the electromagnetic field configuration by means of insertion screws or sliding materials, the change in capacitance and field configuration varying the circuit resonance and thereby tuning the device. One problem with this type of tuning arrangement which depends on mechanical adjustments is the time required to effect the change of frequency. This is a disadvantage in that the mechanical adjustment of the frequency response is considerably time-consuming for applications requiring relatively fast reactions such as in microwave applications where rapid frequency changes are essential for operation of many electronic devices.
Some prior art devices incorporating strip transmission lines use a varactor (voltage variable capacitor) within the circuit itself to change the frequency of operation by varying the varactor capacitance electrically through bias voltage changes. In particular, this type of tuning arrangement has been satisfactory in devices in which a d.c. (direct current) bias is applied to the active elements in the circuit, even in such circuits in which the tuning varactor being included in the circuit itself results in substantial degradation of circuit performance.
However, in many strip transmission line devices having pulsed bias active elements, including oscillators and amplifiers in which a varactor is utilized to frequency tune and phase tune such devices, respectively, certain performance and tuning difficulties have been encountered. The inclusion of a varactor directly into the circuit has tended to significantly degrade the output efficiency and other operating characteristics of pulse-biased devices used in the radio frequency (RF) range, and in particular the microwave region, and the tuning capability has been generally poor.
More specifically, the operating characteristics that are of concern in the application of the present invention include: (1) the RF output power of the device; (2) the bias power to RF power conversion efficiency; (3) the signal-to-noise ratio of the RF output signal; and (4) the quality of the RF spectrum of the output power as compared with the ideal pulsed RF spectrum. A poor quality output, for example, would include an asymmetrical RF spectrum and a spectral display that does not correspond to the ideal ##STR1## envelope expected from a pulsed RF source. These characteristics shall be identified herein as "the operating characteristics" of the device.
Poor operating characteristics of some of the prior art devices utilizing varactor tuning arrangements are the result of the varactor being an integral part of the circuit as by connecting directly the varactor to the strip transmission line. As the varactor capacitance in such arrangements is varied to achieve circuit tuning, the operating characteristics of the device also change. Therefore, these prior art varactor tuning means are not suitable, in particular, for pulsed applications.